Communication subsystem between base stations and base station controllers in burst communication systems

Abstract: Of the type that links a set of base stations (2) with a base station controller (1) by means of unidirectional transmission lines (10). These base stations communicate by radio with at least one mobile terminal (3) making use of time division multiple access techniques. Communications between any base station (2) and its base station controller (1) over unidirectional transmission lines (10) make use of a frame structure with the same bit rate as that of the radio link between the base stations (2) and the mobile terminals (3), and in which the assembly formed by the guard field and the synchronisation field is replaced by a reduced synchronisation field and a control field to carry out the interchange of control and/or signalling information between the base station (2) and its base station controller (1).

Description: OBJECT OF THE INVENTION The invention concerns a communications subsystem between base stations, that form radio links with mobile terminals using TDMA procedures, and base station controllers; this communications subsystem being implemented by means of unidirectional transmission lines. It is of special application in mobile systems and in cordless data transmission and telephony systems, like the DECT system (Digital European Cordless Telecommunications). BACKGROUND TO THE INVENTION In any communications structure for cellular telephony, it is well known how a set of base stations are used to communicate by radio with the mobile terminals that form part of the system, these stations being connected to a base station controller which manages their operation. This is explained in the article "Sistemas de Movilidad Local" by L. Martinez, published in the Revista Española de Electrónica, May 1993, pages 34-39. In this publication various architectures are shown for cordless subscriber loops consisting of mobile terminals, base stations and various types of switches that perform the function of base station controller, for example the cordless private branch exchange of figure 6, page 37, which, in addition to being connected to base stations, also permits connection with standard and extended service telephone sets. The connections between the private branch exchange and the base stations are made, as indicated on page 39 of the article mentioned, by means of twisted wire pairs over which data are transmitted at 2048 kb/s (CCITT Recommendation G-702). Under these circumstances, although not stated in the article mentioned, there is a clear need for an interface conversion which implies the availability of associated subsystems for clock generation, temporary storage memories in which the read/write processes for transmission frequency conversion take place, which, in turn, produces in a small delay that has to be taken into account, etc. As a result, the complexity of the equipment, its cost, and the delays produced in interface conversions that are made in the communications process between the private branch exchanges and the base stations increase considerably. Document COMMUNICATION AND TRANSMISSION, vol.14, no.1, 1992, pages 21 - 28, teaches a base station subsystem architecture at least presents one or more base stations, including the radio and signalling equipment needed to handle traffic within a cell, a base station controller which controls all the cells comprising the area of coverage of the base stations subsystem. The base station manages the physical layer of radio transmission as well as the protocol for the data link the mobile terminals. Depending on the network architecture, one or more base stations can be connected to the base station controller. There are the A-bis interface between the base station and base stations. So, the physical layer of the A-bis interface is defined by the use of 2Mbit/s links, while layer 2 uses a LapD protocol. TECHNICAL PROBLEM TO BE OVERCOME The technical problem to be overcome, consequently, consists in establishing communications between base stations and base station controllers without performing interface conversions. CHARACTERISATION OF THE INVENTION The subsystem according to the invention is applicable at the communications interface between base stations and base station controllers in digital cordless communications systems that make use of time division multiple access techniques for access by the base stations to mobile terminals where the air interface is formed with a frame structure that includes a series of bursts which, in turn, comprise a guard field, a synchronisation field and an information field. These communications between the base stations and the base station controllers are constituted physically by means of unidirectional transmission lines. This subsystem is characterised in that communications over these transmission lines make use of an interface with a frame that has the same bit rate as that of the frame of the air interface of the cordless communications system and its structure is also the same, however in this case the guard field and the synchronisation field are replaced by a reduced synchronisation field to perform the burst synchronisation in this line transmission and by a control field to perform the interchange of control information and/or signalling between the base station controller and the base stations. The clock frequency corresponding to the bit rate used in communications between the base station controller and the actual base station are generated in a single master clock incorporated in the base station controller. It is also characterised in that the control field has a protection mechanism against line errors. The control field includes, at least, two equal subfields, termed control subfields, in which the same control information is sent in duplicate, in a command area, plus one parity control bit for the detection of errors through parity checks, such that if both parity bits, each corresponding to a control subfield, are equal and, in addition, so too are the command areas, then it is considered that there have been no transmission errors. If only one of the parity bits is erroneous, then it is considered that the control subfield with the correct parity bit, is error-free. Finally if both parity bits are erroneous or if both are correct bit the command areas are not equal, then an error message is generated to show that the information sent in the control field could not he recovered. As a consequence of applying the subsystem of the invention, it is not necessary to use an interface that works at a different transmission frequency, which would require the use of many highly complicated, associated interface conversion circuits and the delays implicit in the actual conversion. As a consequence of this and due to the stabilities required in the clock signals, the fact that a single, high stability master clock is used for the set of base stations and their controller means major financial savings. This translates into an important reduction in complexity and costs, as well as meaning that the additional delays are restricted to a very small value. BRIEF FOOTNOTES TO THE FIGURES Below is given a fuller description of the invention based on the following figures in which: figure 1 shows the layout of a mobile communications system in which this invention is applied, and figure 2 shows the configuration of a frame used in a system like the aforementioned, in accordance with the invention. DESCRIPTION OF THE INVENTION The mobile communications system, as indicated in figure 1, is formed from a set of base stations 2, each one with a determined radio coverage to establish cordless communication with at least one mobile terminal 3 by means of the time division multiple access (TDMA) technique and using for this at least one radiofrequency channel, as is the case of the DECT system. The base stations 2 are connected to a base station controller 1 by means of unidirectional transmission lines 10 over which the base stations 2 and their base station controller 1 interchange the necessary control information to set up a communications channel to put a base station 2 into contact with a mobile terminal 3 as well as the actual information transmitted over that channel. To effect the communications between the base stations 2 and the mobile terminals 3 via radio, an air interface is defined that is precisely known from the DECT system specifications, in which each frame with a duration of 10 ms, consists of 24 normal bursts (of 480 bits), there normally being 12 of these for each direction of transmission. Each of the normal bursts mentioned contains, as stated, 480 bits of which at least 56 (60 when the 4-bit Z field is not used) belong to a guard field G during which no signal is transmitted to prevent overlapping between consecutive bursts coming from distinct sources and which may also be at different distances, as well as to permit transmitter on and off times, frequency changes, etc. It also has a synchronisation field S1 of 32 bits which comprises clock synchronising bits and burst synchronising bits, and a data field D of 388 bits that contains the information being interchanged between the base station 2 and the mobile terminal 3. In a similar way, half-length bursts of 240 bits and double-length bursts of 960 bits are defined, so that a normal burst can be replaced by two half-length bursts and two normal bursts can be grouped into one double-length burst. In both cases, these types of burst also have a guard field G and a synchronisation field S1, as well as the data field, which in these cases are of 148 and 868 bits respectively for the half-length and double-length bursts. The links between the base stations 2 and their base station controller 1, formed by means of unidirectional transmission lines 10, are formed through a continuous interface according to the invention, the bit rate of which is the same as that of the air interface and that has a frame structure similar to the latter, employing for this two unidirectional transmission lines 10, one for each transmission direction (base stations to base station controller and base station controller to base stations). The frame structure for these interfaces, as already stated, is similar to that of the air interface but with suppression of the guard field G in each burst, because it is no longer necessary for this type of fixed link, and reducing the synchronisation field S1 of the air interface through the suppression of the bit synchronising part which is not required for continuous transmission, and reducing equally the number of bits in the burst synchronising part. For each of the transmission directions in the unidirectional transmission lines 10, the frames have the same structure. This frame structure holds the same number of bursts, each constituted by a reduced synchronisation field S2 of 8 bits, that contains one fixed word and that serves to determine the beginning of each burst; a control field C of 64 bits per burst that is used to transmit the control and/or signalling information both in the direction of base station controller 1 to base stations 2 and in the direction of base stations 2 to base station controller 1; and the data field F that includes the same information as field D and field Z of a DECT burst. Since the information contained in the control field C is of essential importance for system operation, this field is further protected by means of a protection mechanism against errors based on duplicating information and parity bits. For this, the control field C comprises two control subfields, each of 28 bits and identical to each other, containing respectively a command area CA and CB and a parity control bit PA and PB. In the reception part of the burst in question, the following process takes place: first the parity of each control subfield is obtained and a check is made for coincidence of the respective parity bits PA and PB. If this coincidence is present and also both the command areas CA and CB are equal, then both command areas CA and CB are considered error-free. In the event that only one of the parity bits PA or PB is correct, then that command area CA or CB whose parity bit is correct is considered to be error-free and, finally, if neither of the parity bits PA or PB is correct, or if both are correct but the command areas CA and CB are not equal, then an error message is produced in order to indicate this situation.

Claims: A METHOD FOR COMMUNICATING BETWEEN BASE STATIONS AND BASE STATION CONTROLLERS IN BURST COMMUNICATIONS SYSTEMS, in which each base station (2) establishes a radio link with at least one mobile terminal (3) making use of time division multiple access techniques in the air interface, the frame of which includes a series of bursts that comprise a guard field (G), a synchronisation field (S1) and an information field (D) and in which the communications between base stations (2) and base station controllers (1) are made by means of unidirectional transmission lines (10); characterised in that the method further comprises the steps of: Generating a frame structure with the same bit rate as that of the radio link between said base station (2) and said mobile terminals (3), Replacing of the assembly formed by said guard field (G) and said synchronisation field (S1) by the assembly formed by a reduced synchronisation field (S2) and a control field (C) to carry out the interchange of control and/or signalling information between said base station (2) and its base station controller (1). The method according to claim 1; characterised in that said base station controller (1) includes a clock frequency which generates a single master clock for transmitting information in both directions between said base station controller (1) and said base stations (2). The method according to claim 1; characterised in that said control field (C) includes control and/or signalling information in both the direction from base station controller (1) to base station (2), and in the direction from base station (2) to its base station controller (1). The method according to claim 1; characterised in that said control field (C) includes a protective mechanism against errors in the unidirectional transmission line (10), in said control field (C). The method according to claim 4; characterised in that said control field (C) includes at least two subfields of control information, wherein one is a replica of the other and each includes a command area (CA, CB) that contains the control information and a parity bit (PA, PB) in order to have error detection by means of parity detection; such that: if both parity bits (PA,PB) and both command areas (CA,CB) of both subfields of control information are equal, then both control information subfields (CA,CB) are considered error-free, if only one of the parity bits (PA,PB) of either of the control subfields is correct, then the control subfield with the correct parity bit is considered to be error-free, and if both parity bits (PA,PB) are erroneous, or if both are correct but the command areas (CA and CB) are not equal, then an error message is generated by the part receiving the control field (C) mentioned.

Inventors: ESCRIHUELA LANGA FRANCISCO JOS, HERNANDEZ BLANCO CARLOS, HITA DE LA TORRE CARLOS, MARTINEZ GARCIA FRANCISCO JAVI, ESCRIHUELA LANGA, FRANCISCO JOSE, HERNANDEZ BLANCO, CARLOS, HITA DE LA TORRE, CARLOS, MARTINEZ GARCIA, FRANCISCO JAVIER, Escrihuela Langa, Francisco José, Martinez Garcia, Francisco Jávier
Applicants: CIT ALCATEL, ALCATEL
Application year: 1994
Main classification: H04Q7
IPC codes: H04W88, H04W92